Control circuits are known for controlling DC brushless motors, such as, for example, regulating the rotational speed of DC brushless fan motors that cool the interior of computers. One problem with DC brushless fan motors is that such fan motors traditionally have had a narrow usable input range. Fan speed and input current are approximately proportional to input voltages. Thus, if the input voltage from an unregulated source such as a battery were used to power a DC brushless fan, such as a typical 24 volt nominal battery, the voltage would vary from about 28 volts in float state to about 21 volts in discharged state. This change would cause a DC brushless fan rated at a nominal 3500 RPM to vary as much as about 1000 RPM over the above-mentioned range of battery voltage. Such a large variation in RPM means that the fan is not properly cooling a computer at the low-end of the RPM range, and that power is being wasted at the high-end of the RPM range.
Some DC brushless fan users have multiple input source voltages that their equipment is expected to operate from, with 24 volt and 48 volt systems being the most common. Such multiple source voltages pose the same problem in resultant RPM variation in a DC brushless fan motor as does a single input voltage source whose voltage level widely varies. There is a need to provide a DC brushless fan motor having a high input range with relatively little variation in motor rotational speed. For example, in the telecommunications industry, there is a need to provide a DC brushless fan motor having an input range of about 20-60 volts with little variation in motor rotational speed. However, other input voltage ranges may be provided for other motor applications.
Linear regulators have been used to regulate DC brushless fan rotational speed. However, the linear regulator approach poses an efficiency problem. A DC brushless fan that draws 18 watts at 21 volts will draw almost 27 watts when operating at 28 volts, and 54 watts at 56 volts input, with the increase in power draw having to be dissipated as heat.
Pulse-width modulation (pwm) has also been used in the prior art to regulate motor speed. One method commonly used is to pulse-width modulate the commutation transistors to the DC brushless motor. This employment of pulse-width modulation reduces the dissipation of energy involved with changing motor speed. However, pulse-width modulating the commutation transistors does not permit large changes in input voltage without widely-varying the rotational speed of the DC brushless motor. This method is most commonly used in thermal DC brushless fans to reduce DC brushless fan speed at low temperatures. The speed variation is unfortunately even wider than that of the non-speed controlled type, and clamp dissipation is still relatively high.
Another pwm approach is to use a full bridge driver. This involves placing a bipolar motor winding between the legs of four switching transistors and controlling the timing of the pwm modulator and commutation logic to regulate motor current. Wide input voltage ranges are possible with high efficiency. A well designed full bridge driver can regulate motor speed over a better than 3:1 range of input voltage. The chief drawbacks are complicated logic and the difficulties of driving the four switching transistors without cross conduction through the series connected pairs. Although many manufacturers offer integrated full bridge devices, most suffer from a limitation of current and/or voltage.
Another approach is to employ a pwm switching voltage regulator to accommodate a wide range of input voltages without widely varying the rotational speed of the motor. However, this requires relatively bulky filter inductors and capacitors.
Of the above-mentioned pwm approaches, the pwm voltage regulator regulates motor voltage. The other methods typically regulate motor current. Voltage regulation is preferred to minimize variations in desired DC brushless motor speed. In other words, the variation in motor speed from motor to motor for a given current is greater than the variation in motor speed for a given voltage. Additionally, motor torque is a function of motor current. Therefore, if motor current is reduced in order to reduce motor speed to a low value, the motor torque becomes low. This means that the motor speed is sensitive to applied load (i.e. back pressure). This sensitivity to back pressure results in large speed deviations from the desired value. Motor-starting at low desired speeds is also a problem in that if the motor current is set too low then the motor will not be able to overcome the magnetic detents used to position the rotor away from the null point.
The invention uses the advantages of a pwm voltage regulator, wide speed control, wide input voltage range capability, and high power efficiency while eliminating the large filtering components such as bulky capacitors and inductors interfacing the voltage regulator with a DC brushless motor.
It is therefore an object of the present invention to provide a DC brushless motor regulator which handles a relatively wide range of input voltages with little variation in the rotational speed of the motor.
It is another object of the present invention to provide a DC brushless motor regulator that eliminates the relatively bulky filter capacitors and inductors interfacing the regulator and motor.
The above and other objects and advantages of this invention will become more readily apparent when the following description is read in conjunction with the accompanying drawings.